U.S. patent application number 12/870959 was filed with the patent office on 2011-03-24 for novel metal air cathode manganese oxide contained in octahedral molecular sieve.
This patent application is currently assigned to ROVCAL, INC.. Invention is credited to Denis D. Carpenter, Akshaya Kumar Padhi, Jeffrey A. Poirier.
Application Number | 20110070487 12/870959 |
Document ID | / |
Family ID | 44677776 |
Filed Date | 2011-03-24 |
United States Patent
Application |
20110070487 |
Kind Code |
A1 |
Padhi; Akshaya Kumar ; et
al. |
March 24, 2011 |
NOVEL METAL AIR CATHODE MANGANESE OXIDE CONTAINED IN OCTAHEDRAL
MOLECULAR SIEVE
Abstract
An oxygen reduction electrode, e.g., an air cathode, comprising
manganese oxides having octahedral molecular sieve structures as
active catalyst materials and use of such an electrode as a
component of a metal-air cell.
Inventors: |
Padhi; Akshaya Kumar;
(Madison, WI) ; Carpenter; Denis D.; (Verona,
WI) ; Poirier; Jeffrey A.; (Madison, WI) |
Assignee: |
ROVCAL, INC.
Madison
WI
|
Family ID: |
44677776 |
Appl. No.: |
12/870959 |
Filed: |
August 30, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11272990 |
Nov 14, 2005 |
|
|
|
12870959 |
|
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|
Current U.S.
Class: |
429/206 |
Current CPC
Class: |
B01J 35/1014 20130101;
C01P 2006/12 20130101; H01M 4/623 20130101; H01M 12/06 20130101;
H01M 4/364 20130101; H01M 2300/0085 20130101; Y02E 60/10 20130101;
H01M 2300/0014 20130101; B01J 23/8892 20130101; H01M 4/043
20130101; C01P 2002/54 20130101; H01M 4/625 20130101; H01M 10/24
20130101; C01G 45/02 20130101; H01M 4/50 20130101; H01M 4/9016
20130101; C01P 2002/72 20130101; Y02E 60/50 20130101; C01P 2004/41
20130101; C01P 2006/10 20130101; H01M 4/96 20130101; C01P 2006/40
20130101; B01J 23/34 20130101; H01M 4/0471 20130101 |
Class at
Publication: |
429/206 |
International
Class: |
H01M 10/26 20060101
H01M010/26 |
Claims
1. An electrochemical cell comprising: an anode; an aqueous
alkaline electrolytic solution; and an oxygen reduction electrode
comprising a mixed manganese oxide catalyst comprising an
octahedral molecular sieve structure and Mn.sub.2O.sub.3, wherein
the octahedral molecular sieve structure and the Mn.sub.2O.sub.3
are present in the mixed catalyst in a ratio between about 1:9 and
about 9:1.
2. An electrochemical cell as set forth in claim 1 wherein the
manganese in the mixed catalyst has an average oxidation state of
less than about 3.7.
3. An electrochemical cell as set forth in claim 2 wherein the
manganese in the mixed catalyst has an average oxidation state
between about 3.1 and 3.7.
4. An electrochemical cell as set forth in claim 3 wherein the
manganese in the mixed catalyst has an average oxidation state of
about 3.3.
5. An electrochemical cell as set forth in claim 1 wherein the
mixed catalyst has been calcined at a temperature between about
450.degree. C. and about 650.degree. C.
6. An electrochemical cell as set forth in claim 5 wherein the
mixed catalyst has been calcined at a temperature between about
500.degree. C. and about 600.degree. C.
7. An electrochemical cell as set forth in claim 6 wherein the
mixed catalyst has been calcined at a temperature of about
550.degree. C.
8. An electrochemical cell as set forth in claim 1 wherein the
octahedral molecular sieve structure and the Mn.sub.2O.sub.3 are
present in the mixed catalyst in a ratio of between about 1.27:1
and about 1:1.
9. An electrochemical cell as set forth in claim 8 wherein the
octahedral molecular sieve structure and the Mn.sub.2O.sub.3 are
present in the mixed catalyst in a ratio of about 1:1.
10. An electrochemical cell as set forth in claim 1 wherein the
manganese oxide contained in the mixed catalyst comprises particles
having a B.E.T. surface area between about 10 m.sup.2/g and about
100 m.sup.2/g.
11. An electrochemical cell as set forth in claim 10 wherein the
manganese oxide contained in the mixed catalyst comprises particles
having a B.E.T. surface area between about 15 m.sup.2/g and about
90 m.sup.2/g.
12. An electrochemical cell as set forth in claim 1 wherein the
manganese oxide contained in the mixed catalyst comprises particles
having a mean particle size between about 0.5 .mu.m and about 2.0
.mu.m.
13. An electrochemical cell as set forth in claim 12 wherein the
manganese oxide contained in the mixed catalyst comprises particles
having a mean particle size between about 1.5 .mu.m and 2.0
.mu.m.
14. An electrochemical cell comprising: an anode; an aqueous
alkaline electrolytic solution; and an oxygen reduction electrode
comprising a mixed manganese oxide catalyst comprising an
octahedral molecular sieve structure and Mn.sub.2O.sub.3, wherein
the manganese in the mixed catalyst has an average oxidation state
of less than about 3.7.
15. An electrochemical cell as set forth in claim 14 wherein the
manganese in the mixed catalyst has an average oxidation state
between about 3.1 and 3.7.
16. An electrochemical cell as set forth in claim 14 wherein the
mixed catalyst has been calcined at a temperature between about
450.degree. C. and about 650.degree. C.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part application of
U.S. patent application Ser. No. 11/272,990 which was filed on Nov.
14, 2005 and titled "NOVEL METAL AIR CATHODE: MANGANESE OXIDE
CONTAINED IN OCTAHEDRAL MOLECULAR SIEVE", the contents of which are
incorporated by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates generally to electrochemical
cells such as batteries. More particularly, the present invention
relates to an oxygen reduction electrode, e.g., an air cathode,
comprising manganese oxide contained in an octahedral molecular
sieve structure as an active catalyst material and use of such an
electrode as a component of a metal-air cell.
BACKGROUND OF THE INVENTION
[0003] Among the various metal oxide catalyst materials for use as
cathode components in metal-air cells, manganese oxide catalysts
have been investigated due to their low cost and high catalytic
activity for oxygen reduction. Manganese can exhibit a number of
different oxidation states. Due to the stability of these oxidation
states, including 2+, 3+ and 4+, a single composition may contain a
stable mixture of a variety of different oxides such as, e.g.,
MnO.sub.2, Mn.sub.2O.sub.3, Mn.sub.3O.sub.4, and MnO. The use of
manganese oxides, in a variety of oxidation states, as an active
catalyst material of a cathode has been reported. See U.S. Pat. No.
4,595,643 (Koshiba et al.) and U.S. Pat. No. 4,892,637 (Sauer et
al).
[0004] Manganese oxides when used in electrochemical cells have
presented several challenges as active catalyst materials. Chief
among those challenges is limited catalytic activity. Higher
voltage in the metal-air cell depends upon rapid oxygen reduction
at the cathode. Such oxygen reduction kinetics, being catalyst
limited, allow the cells to be used for low or moderate power
applications, such as hearing aids for users with moderate hearing
loss, but limits their use in more demanding high power
applications.
[0005] Foremost among the factors causing polarization in cells
employing manganese oxides as active catalyst material is the
buildup of peroxides at the electrode due to slow reaction kinetics
of both reduction of peroxides to hydroxyl ions and decomposition
of peroxides to water and oxygen at the cathode. During manganese
oxide catalyzed reduction of oxygen, peroxides, which form upon
oxygen reduction, may either decompose into water and adsorbed
oxygen or undergo additional reduction to hydroxyl ions, the
desired redox reaction. Slow or wasteful decomposition inhibits the
desired reduction reaction from peroxides to hydroxyl ions, which
can lower the voltage produced by conventional metal air cells.
[0006] Manganese oxide contained in an octahedral molecular sieve
structure is an efficient catalyst for the decomposition of
hydrogen peroxide. See Zhou et al., Journal of Catalysis, 176,
321-328 (1998). A catalyst which efficiently removes hydrogen
peroxide may be advantageous because the peroxide is an
intermediate in the reduction path of oxygen to hydroxyl ions. This
advantage allows the cell voltage to better reflect the full
potential of the oxygen reduction half cell reaction for a given
current density, thus resulting in a high power cathode. Recently,
manganese oxides contained in an octahedral molecular sieve
structure such as the hollandite structure have been explored
because of their high catalytic activity for peroxide reduction.
Zhang and Zhang describe an all solid-state galvanic cell having a
cathode comprising nanostructured MnO.sub.2/mesocarbon microbeads
(MCMB), a compacted zinc anode made from Zn powder and a PTFE
binder, and a polymer gel electrolyte comprising a potassium salt
of poly(acrylic acid). See "MnO.sub.2/MCMB electrocatalyst for all
solid-state alkaline zinc-air cells," Electrochemica Acta, 49
(2004) 873-877. The composition of the MnO.sub.2 component of the
MnO.sub.2/MCMB cathode is said to have been KMn.sub.8O.sub.16 and
the authors report that XRD analysis indicated that it had a
hollandite structure. In preparation of the cathode, a 4:1 wt./wt.
MnO.sub.x/MCMB composite was synthesized by preparing a suspension
of MCMB (specific surface area=3.5 m.sup.2/g) and a solution of
MnSO.sub.4.H.sub.2O in distilled water. K.sub.2S.sub.2O.sub.8 was
added and the mixture refluxed until the pH decreased to about 0.5.
The cathode of Zhang and Zhang was tested under idealized
conditions, such that its practical use in a commercial battery is
difficult to determine, i.e., the cathode was tested in a flooded
half-cell and in the presence of a large excess of KOH electrolytic
solution.
SUMMARY OF THE INVENTION
[0007] The present disclosure is directed to an electrochemical
cell comprising an anode, an aqueous alkaline electrolytic solution
and an oxygen reduction electrode. The oxygen reduction electrode
comprises a mixed manganese oxide catalyst comprising an octahedral
molecular sieve structure and Mn.sub.2O.sub.3, wherein the
octahedral molecular sieve structure and the Mn.sub.2O.sub.3 are
present in the mixed catalyst in a ratio between about 1:9 and
about 9:1.
[0008] The present disclosure is also directed to an
electrochemical cell comprising an anode, an aqueous alkaline
electrolytic solution and an oxygen reduction electrode. The oxygen
reduction electrode comprises a mixed manganese oxide catalyst
comprising an octahedral molecular sieve structure and
Mn.sub.2O.sub.3, wherein the manganese in the mixed catalyst has an
average oxidation state of less than about 3.7.
[0009] Other aspects of this invention will be in part apparent and
in part pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a cross-section of a conventional metal-air
cell.
[0011] FIG. 2 is a schematic of the structure of manganese oxide
contained in an octahedral molecular sieve structure with a metal
cation residing in an atomic scale tunnel.
[0012] FIG. 3 is a flow chart showing the entire preparation of a
cathode according to the present invention.
[0013] FIG. 4 shows X-ray powder diffraction (XRD) patterns of
cryptomelanes. The bottom pattern 70 shows empirically determined
X-ray diffraction peaks corresponding to cryptomelane, which had
been dried at 95.degree. C. This bottom pattern is overlaid with
the standard PDF pattern for cryptomelane,
M-K.sub.2-xMn.sub.2O.sub.16 (44-1386), which is indicated by the
narrow lines. The PDF patterns (powder diffraction files) are
available as a database. The top pattern 68 shows empirically
determined X-ray diffraction peaks corresponding to cryptomelane,
which had been calcined at 300.degree. C. The XRD pattern of the
material of the present invention matched closely to that of PDF
(44-1386) corresponding to M-K.sub.2-xMn.sub.8O.sub.16.
[0014] FIG. 5 shows X-ray powder diffraction (XRD) patterns of
cryptomelanes. The bottom pattern 77 shows empirically determined
X-ray diffraction peaks corresponding to cryptomelane, which had
been calcined at 300.degree. C. This bottom pattern is overlaid
with the standard PDF pattern for cryptomelane,
M-K.sub.2-xMn.sub.8O.sub.16 (44-1386), which is indicated by the
narrow lines. The top pattern 72 shows empirically determined X-ray
diffraction peaks corresponding to Ni-exchanged cryptomelane, which
had been calcined at 300.degree. C.
[0015] FIG. 6 shows X-ray powder diffraction (XRD) patterns of
cryptomelanes. The bottom pattern 80 shows empirically determined
X-ray diffraction peaks corresponding to Ni-exchanged cryptomelane
dried at 95.degree. C. This bottom pattern is overlaid with the
standard pattern for cryptomelane, M-K.sub.2-xMn.sub.8O.sub.16
(44-1386), which is indicated by the narrow lines. The middle
pattern 82 shows empirically determined X-ray diffraction peaks
corresponding to Ni-exchanged cryptomelane dried at 120.degree. C.
The top pattern 84 shows empirically determined X-ray diffraction
peaks corresponding to Ni-exchanged cryptomelane calcined at
400.degree. C.
[0016] FIG. 7 shows X-ray powder diffraction (XRD) patterns of
cryptomelanes. The bottom pattern 92 shows empirically determined
X-ray diffraction peaks corresponding to Ni-exchanged cryptomelane
calcined at 300.degree. C. This bottom pattern is overlaid with the
standard pattern for cryptomelane, M-K.sub.2-xMn.sub.8O.sub.16
(44-1386), which is indicated by the narrow lines. The top pattern
90 shows empirically determined X-ray diffraction peaks
corresponding to Ni-exchanged cryptomelane calcined at 300.degree.
C., which has been additionally subjected to 16 hours of stirring
in KOH electrolyte.
[0017] FIG. 8 shows polarization curves for: a cathode loaded with
Ni-exchanged cryptomelane, a cathode loaded with heat-treated
cryptomelane in combination with catalytically active manganese
oxide compounds, and a conventional cathode.
[0018] FIG. 9 shows discharge voltages of: a cathode loaded with
Ni-exchanged cryptomelane, a cathode loaded with heat-treated
cryptomelane in combination with catalytically active manganese
oxide compounds, and a conventional cathode.
[0019] FIG. 10 displays the effect of temperature of calcination on
the manganese oxidation state and the surface area of a
catalyst.
[0020] FIGS. 11A-11C depict the Rietveld analysis for X-ray
diffraction data for samples calcined at 500.degree. C.,
550.degree. C. and 600.degree. C.
[0021] FIG. 12 displays the discharge and recovery curves for gas
diffusion electrodes with catalysts made at 450.degree. C.,
500.degree. C., 550.degree. C., 600.degree. C. and 650.degree.
C.
[0022] FIG. 13 displays the discharge voltages at 3 minutes at 10
mA/cm.sup.2 for gas diffusion electrodes with catalysts made at
450.degree. C., 500.degree. C., 550.degree. C., 600.degree. C. and
650.degree. C.
[0023] FIG. 14 displays the recovery voltages at 5 minutes into
rest (offload) for gas diffusion electrodes with catalysts made at
450.degree. C., 500.degree. C., 550.degree. C., 600.degree. C. and
650.degree. C.
[0024] FIG. 15 displays the polarization curves for gas diffusion
electrodes with catalysts made at 450.degree. C., 500.degree. C.,
550.degree. C., 600.degree. C. and 650.degree. C.
[0025] FIG. 16 displays the thermal gravimetric study of the
manganese oxide having an octahedral molecular sieve structure
(cryptomelane) in air and argon.
[0026] FIG. 17 displays the change in the manganese oxidation state
and the surface area with heat treatment temperature.
[0027] FIG. 18 displays the X-ray diffraction pattern of
cryptomelane samples heated at 120.degree. C., 300.degree. C.,
400.degree. C., 500.degree. C. and 550.degree. C.
[0028] FIG. 19 displays the open circuit voltage of zinc air cells
with cathodes comprising catalysts made at 120.degree. C.,
300.degree. C. and 500.degree. C.
[0029] FIG. 20 displays 620 ohm constant load discharge curves for
zinc air cells comprising cathodes made with catalysts made at
120.degree. C., 300.degree. C. and 500.degree. C.
[0030] FIG. 21 displays 374 ohm constant load discharge curves for
zinc air cells made with cathodes comprising catalysts made at
120.degree. C., 300.degree. C. and 500.degree. C.
[0031] FIG. 22 displays 374 ohm constant load discharge curves for
zinc air cells stored for 20 days at 60.degree. C. with cathodes
comprising catalysts made at 120.degree. C., 300.degree. C. and
500.degree. C.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] In accordance with this invention, a mixed manganese oxide
comprising an octahedral molecular sieve structure and
Mn.sub.2O.sub.3 serves as an oxygen reduction catalyst in a novel
oxygen reduction electrode (i.e., cathode). The cathode of the
present invention has application in a wide variety of
electrochemical cells. It is especially advantageous in galvanic
cells, such as in metal-air cells, and more specifically, in a
zinc-air cell. Metal-air cells comprising the novel cathode may
usefully be constructed as button cells for the various
applications in which this type of cell has been found attractive.
It shall be understood, however, that the present invention has
application to electrochemical cells other than button cells. For
example, the cathode active material of the present invention may
find application in any metal air cell using flat, bent, or
cylindrical electrodes. Among the cylindrical metal-air cells, the
cathode active material is applicable to those shaped for AA, AAA,
AAAA, C, and D cells.
Metal-Air Cells
[0033] Referring now to FIG. 1, a cross-section of a metal-air cell
1 is shown. The cell comprises an electrically conductive anode can
5 received in an electrically conductive cathode can 10. A
generally cylindrical or annular thin-walled dielectric grommet 25
(also referred to as a gasket or seal) electrically insulates the
anode can 5 from direct electrical contact with the cathode can 10
and forms a seal therebetween to close the reactive materials
within the cell 1.
[0034] Anode material 6, which comprises active metal, typically
present as a powder, and an ionically conductive, preferably
gelled, electrolytic solution are contained within the anode can 5
in electrical contact therewith. The anode, which is referred to as
the negative terminal, is the site of oxidation. Preferably, the
anode material comprises an electropositive material such as zinc,
lithium, or aluminum metal. Preferably, the ionically conductive
electrolytic solution is an aqueous gel comprising alkaline salt,
such as potassium hydroxide, sodium hydroxide, or lithium
hydroxide.
[0035] Air cathode material 14, which typically comprises a porous,
highly conductive material, for example carbon powder, combined
with one or more electrocatalytically active materials,
conventionally manganese dioxide, is contained within the cathode
can 10 in electrical contact therewith. According to the present
invention, the electrocatalytically active material comprises a
mixed manganese oxide comprising an octahedral molecular sieve
structure and Mn.sub.2O.sub.3. The air cathode, which is referred
to as the positive terminal, is the site of reduction. In a
metal-air cell, the cathode can 10 comprises openings 19, which
allow atmospheric oxygen to diffuse into the interior of the
cathode can, through a diffusion layer 21, and come into electrical
contact with the air cathode material 14, where the oxygen is
reduced to hydroxyl ions. Oxidation of the anode material 6
provides electrons for the reduction of oxygen. The anode can and
cathode can are electrically insulated by the dielectric grommet
25. Therefore, the electrical circuit may be completed through a
wire (not shown), which may carry electrons through a load (not
shown), from the anode can 5 to the cathode can 10. The cathode can
10 is in electrical contact with a current collector 23 which
collects the electrons for the reduction of oxygen at the air
cathode material 14. A separator 18, which is a permeable membrane
or a porous film having electrolytic solution within the pores,
allows the passage of hydroxyl ions from the air cathode material
14 into the ionically conductive electrolytic solution for transfer
to the anode but otherwise prevents electrical contact between the
air cathode material 14 and the anode material 6.
Zinc-Air Cells
[0036] A typical example of a metal-air cell is a zinc-air cell. In
a zinc-air cell, the air cathode comprises an electrically
conductive material which is air porous. This porosity allows
oxygen, which has diffused through the openings in the cathode can,
to come into electrical contact with the air cathode material,
where it is reduced to hydroxyl ions. The half cell reaction
is:
O.sub.2+2H.sub.2O+4e.sup.-=>4OH.sup.- E.sup.o=0.40V v. S.H.E.
(1)
[0037] Air cathodes may comprise active catalyst materials,
hydrophobic polymeric binders, and carbon powders. The air cathode
suitably comprises between about 0.1% and about 90% by mass
manganese oxide active catalyst material, more suitably between
about 10% and about 50%, still more suitably between about 25% and
about 40%. The air cathode suitably comprises between about 1% and
about 50% hydrophobic polymeric binder, more suitably between about
5% and about 30%, still more suitably between about 10% and about
25%.
[0038] Carbon powders are typically characterized as electrically
conductive carbon powders and catalytically active powders.
Characterizing a particular carbon powder as either electrically
conductive or catalytically active is not meant to limit its
utility in the cell. For example, it is known that electrically
conductive carbon powders have some catalytic activity for oxygen
reduction. It is also known that catalytically active carbon
powders are also electrically conductive. Typical electrically
conductive carbon powders include graphite, carbon black, and
combinations thereof. In those embodiments where an electrically
conductive carbon powder is present, the air cathode suitably
comprises between about 1% and about 40% by mass electrically
conductive carbon powder, more suitably between about 1% and about
15%, still more suitably between about 1% and about 5%.
Catalytically active carbon powders include activated carbon. The
air cathode suitably comprises between about 10% and about 90% by
mass catalytically active carbon powders, more suitably between
about 30% and about 85%, still more suitably about 50%. Activated
carbon has been found to have catalytic activity for the oxygen and
hydrogen peroxide reduction and oxygen adsorptive properties.
Suitably, between about 20% and about 90%, more suitably between
about 45% and about 80%, still more suitably between about 55% and
about 75% and even more suitably between about 65% and about 75% by
weight of all of the carbon powders (catalytic and conductive) have
particle sizes smaller than 325 mesh. The carbon powders suitably
have a total B.E.T. surface area between about 200 m.sup.2/g and
about 2000 m.sup.2/g, more suitably between about 500 m.sup.2/g and
about 1200 m.sup.2/g. Preferably, the carbon powders are porous.
With particular regard to activated carbon powders, the activated
carbon powders suitably have a B.E.T. surface area between about
900 m.sup.2/g and about 1200 m.sup.2/g, more suitably between about
1000 m.sup.2/g and about 1100 m.sup.2/g.
[0039] An exemplary manganese oxide compound which is catalytically
active is described by Passaniti et al. See U.S. Pat. Nos.
5,308,711 and 5,378,562, both of which are hereby incorporated by
reference in their entirety. In Passaniti et al., the inventors
disclosed that PWA activated carbon introduced into an alkali metal
permanganate solution would cause a reaction at room temperature
which forms insoluble, catalytically active manganese oxide
compounds. The activated carbon reduces the permanganate to
catalytically active manganese oxide compounds. It was discovered
that electrodes comprising activated carbon and the catalytically
active manganese oxide compounds operated at higher potentials over
the full range of tested current densities as compared to
electrodes comprising conventional catalysts. The catalytically
active manganese oxide compounds may be obtained by contacting
alkali metal permanganate with activated carbon in a weight ratio
of alkali metal permanganate to carbon between about 0.01:1 and
about 0.2:1, more suitably between about 0.02:1 and about 0.1:1,
and even more suitably between about 0.04:1 and about 0.08:1. The
resulting product, which comprises crystalline or amorphous
manganese oxide on a carbon support, is an effective catalyst for
the reduction of oxygen. Preferable sources of permanganate include
alkali metal permanganate salts such as permanganate salts of
lithium, sodium, and potassium.
[0040] According to the present invention, the air cathode
comprises a mixed manganese oxide comprising an octahedral
molecular sieve structure and Mn.sub.2O.sub.3 as a catalytically
active material. Alternatively, the air cathode may comprise
manganese oxide contained in an octahedral molecular sieve
structure in combination with a manganese oxide compound in another
form, such as, for example, the catalytically active manganese
oxide compounds described by Passaniti et al. It has been found
that manganese oxide contained in an octahedral molecular sieve
structure has catalytic activity for the decomposition and
reduction of peroxide, an intermediate in oxygen reduction.
Octahedral molecular sieves encompass a class of oxides of metals
characterized by a particular framework structure comprising
MO.sub.6 octahedra, wherein M represents a metal cation. M may be
Mn, Fe, Ti, V, Zr, Sn, Pb, Ge, or Cr in some of the known
octahedral molecular sieve structures. The framework structure
comprises a particular arrangement of MO.sub.6 octahedra forming
atomic scale tunnels. For example, an octahedral molecular sieve
comprises a framework of metal oxides contained in MO.sub.6
octahedra linked at vertices and edges into a 2.times.2 arrangement
forming atomic scale tunnels having dimensions of approximately 4.6
.ANG..times.4.6 .ANG.. In another example, an octahedral molecular
sieve comprises a framework of metal oxides contained in MO.sub.6
octahedra linked at vertices and edges into a 3.times.3 arrangement
forming atomic scale tunnels having dimensions of approximately 6.9
.ANG..times.6.9 .ANG.. Other arrangements are known. See DeGuzman,
Roberto N. et al., Chem. Mater. 1994, 6, 815-821 (the disclosure of
which is hereby incorporated in its entirety).
[0041] A manganese oxide contained in octahedral molecular sieve
structure having a 2.times.2 arrangement of MnO.sub.6 octahedra is
depicted generally in FIG. 2. The octahedral molecular sieve 30 is
characterized by atomic scale tunnels having dimensions of 4.6
.ANG..times.4.6 .ANG. due to a particular framework of MnO.sub.6
octahedra 32 linked at vertices and edges into a 2.times.2
arrangement. The MnO.sub.6 octahedra, being in 2.times.2 framework,
have an approximate stoichiometry of Mn.sub.8O.sub.16 surrounding
each atomic scale tunnel. A metal cation 34, such as K.sup.+,
Ba.sup.2+, Pb.sup.2+, Sr.sup.2+, Cu.sup.2+, Mg.sup.2+, Fe.sup.3+,
Co.sup.2+, and Ni.sup.2+, may reside in the atomic scale tunnels.
Several manganese oxides contained in octahedral molecular sieve
structures are known. For example, manganese oxides arranged in a
2.times.2 framework forming atomic scale tunnels in which Ba.sup.2+
ions reside in the atomic scale tunnels are known as hollandites.
Hollandite has an approximate stoichiometry of
Ba(Mn.sup.4+,Mn.sup.3+,Mn.sup.2+).sub.8O.sub.16. Where Pb.sup.2+
ions reside in the atomic scale tunnels, the structure is known as
coronadite. Coronadite has an approximate stoichiometry of
Pb(Mn.sup.4+,Mn.sup.3+,Mn.sup.2+).sub.8O.sub.16. Where K.sup.+ ions
reside in the atomic scale tunnels, the structure is known as
cryptomelane. Cryptomelane has an approximate stoichiometry of
K(Mn.sup.4+,Mn.sup.3+,Mn.sup.2+).sub.8O.sub.16. Where Sr.sup.2+
ions reside in the atomic scale tunnels, the structure is known as
strontiomelane. Strontiomelane has an approximate stoichiometry of
Sr(Mn.sup.4+,Mn.sup.3+,Mn.sup.2+)O.sub.16. Manjiroite refers to a
manganese oxide contained in an octahedral molecular sieve
structure in which K.sup.+ and Na.sup.+ ions reside in the atomic
scale tunnels. Manjiroite has an approximate stoichiometry of
Na,K(Mn.sup.4+,Mn.sup.3+,Mn.sup.2+).sub.8O.sub.16. In each of the
above formulae, a composition can contain manganese in a variety of
oxidation states.
[0042] Manganese oxide contained in an octahedral molecular sieve
structure can be synthesized such that substantially no metal ions
reside in the atomic scale tunnels immediately after a product is
obtained, as in Example 1 of Lecerf et al. (U.S. Pat. No.
4,975,346). Exposure to a metal cation solution places metal
cations into the unoccupied tunnels. Manganese oxide contained in
an octahedral molecular sieve structure formed by this method may
have the empirical formula:
M.sub.YMnO.sub.Z
wherein M is a metal cation residing in the atomic scale tunnel.
The metal cation may be K.sup.+, Ba.sup.2+, Pb.sup.2+, Sr.sup.2+,
Cu.sup.2+, Mg.sup.2+, Fe.sup.3+, Co.sup.2+, and Ni.sup.2+. Y
reflects the proportion of metal cation which may have a value
between about 0.01 and about 0.25, suitably between about 0.05 and
about 0.15; and Z typically has a value between about 1.8 and about
2.2, suitably between about 1.9 and about 2.0. For example, a
manganese oxide contained in an octahedral molecular sieve
structure may be synthesized having no metal ions residing in the
tunnel. By exposing this manganese oxide to a nickel ion containing
solution, an octahedral molecular sieve structure is formed which
has an empirical formula of:
Ni.sub.RMnO.sub.Z
wherein R reflects the proportion of nickel cation in the
structure, and the value of R typically is between about 0.01 and
about 0.20, suitably between about 0.05 and about 0.15. Z typically
has a value between about 1.8 and about 2.2, suitably between about
1.9 and about 2.0.
[0043] It is also known that a particular manganese oxide contained
in an octahedral molecular sieve structure may be treated to
partially exchange metal ions already present in the tunnels with
different metal ions. For example, a metal-exchanged cryptomelane
may be synthesized by exchanging a portion of the potassium ions
resident in the atomic scale tunnels with another metal ion.
Generally, the potassium ions residing in the tunnels of
cryptomelane can be readily exchanged with any of a variety of
metal ions, such as Cu.sup.2+, Mg.sup.2+, Fe.sup.3+, Co.sup.2+, and
Ni.sup.2+, with nickel being currently preferred. The exchange
reaction results in a cryptomelane having an empirical formula:
M.sub.YK.sub.(X-vY)MnO.sub.Z
wherein M is a metal cation which may be Cu.sup.2+, Mg.sup.2+,
Fe.sup.3+, Co.sup.2+, or Ni.sup.2+; X is the proportion of
potassium present before the exchange reaction which may be between
0 and about 0.25; Y reflects the proportion of metal cation
replacing potassium which may be between about 0.01 and about 0.1;
v is the oxidation state of the metal cation which replaces
potassium ion; and Z typically has a value between about 1.9 and
about 2.2, suitably about 1.95. Where the exchanged cation is
nickel, for example, the cryptomelane may be referred to as
"nickel-exchanged cryptomelane". By controlling the concentration
and exposure time, the extent of exchange may be limited to
replacing about 10% of the potassium ions in the atomic scale
tunnels. Alternatively, conditions can be varied to replace between
about 40% and about 50% of the potassium ions with nickel
cations.
[0044] In a preferred range of embodiments, the manganese oxide
contained in an octahedral molecular sieve structure is
cryptomelane, i.e., potassium ions reside in the atomic scale
tunnel. In such embodiments, the atomic ratio of Mn to K in the
cryptomelane is suitably greater than 4:1, typically between about
5:1 and about 15:1. In various preferred embodiments, the
cryptomelane may be substantially devoid of metals other than Mn
and K. For example, in such embodiments, the atomic ratio of oxygen
to the sum of all metals other than Mn, i.e., the ratio of Z/X in
the below formula, may be at least about 5, typically between about
7 and about 220. In such preferred embodiments, the cryptomelane
has an empirical formula approximating:
K.sub.XMnO.sub.Z
wherein X may be between 0 and about 0.25, and Z is typically less
than about 2.15, more suitably between about 1.8 and about 2.15,
and more suitably about 1.9 to about 1.95.
[0045] In a manganese oxide contained in an octahedral molecular
sieve structure, the oxidation state of the manganese can typically
range from about 3.6 to about 4.2. In a preferred range of
embodiments, the oxidation state of the manganese can range from
about 3.9 to about 4.2. In another preferred range of embodiments,
the octahedral molecular sieve structure of the novel cathode is
preferably calcined, which can lower the oxidation state of the
manganese contained therein. Accordingly, the oxidation state of
the Mn contained therein is suitably not greater than about 3.85,
more suitably between about 3.60 and about 3.85, and even more
suitably between about 3.70 and 3.85.
[0046] Preferably, the manganese oxide contained in an octahedral
molecular sieve structure has a mean particle size between about
0.01 .mu.m and about 50 .mu.m and a B.E.T. surface area between
about 50 m.sup.2/g and about 500 m.sup.2/g. In a preferred
embodiment of the present disclosure, the manganese oxide contained
in the mixed catalyst material typically has an average (i.e.,
mean) particle size between about 0.5 .mu.m and about 2.0 .mu.m,
more suitably between about 1.5 .mu.m and about 2.0 .mu.m, more
suitably between about 1.7 .mu.m and bout 1.9 .mu.m and even more
suitably of about 1.8 .mu.m. In another preferred embodiment, the
manganese oxide contained in the mixed catalyst material typically
comprises particles having a B.E.T. surface area between about 10
m.sup.2/g and about 100 m.sup.2/g, more suitably between about 15
m.sup.2/g and about 90 m.sup.2/g, more suitably between about 16
m.sup.2/g and about 45 m.sup.2/g and even more suitably of about 28
m.sup.2/g.
[0047] The octahedral molecular sieve material can be calcined or
non-calcined. As generally detailed elsewhere herein, "drying"
typically refers to heating a material at a temperature of less
than about 300.degree. C. for a given period of time, while
"calcining" typically refers to heating a material at a temperature
of about 300.degree. C. or higher for a given period of time. As a
calcined material, i.e., heat-treated at a temperature of about
300.degree. C. or higher, the octahedral molecular sieve structure
of the novel cathode also preferably has low total water content,
e.g., suitably less than about 10 wt. %, more suitably less than
about 4 wt. %, still more suitably less than about 2 wt. %. The
term "heat-treated cryptomelane" can be used to refer to a
cryptomelane that has been heat-treated, suitably at a temperature
between about 95.degree. C. and about 600.degree. C. (or within
another exemplary range as further detailed elsewhere herein),
suitably for between about 1 hour and about 5 hours, for example,
under static conditions, such as in a muffle furnace, or for a time
less than one hour under dynamic conditions, for example, in a
rotary kiln or fluid bed. Under reduced pressure conditions, drying
can be effected at lower temperatures, for example at 75.degree. C.
to 100.degree. C. in a vacuum oven at, for example, about -29 in Hg
(about 3.4 kPa) to about -26 in Hg (about 13.6 kPa).
[0048] In a preferred embodiment of the present disclosure, the
electrochemical cell comprises an anode, an aqueous alkaline
electrolytic solution, and an oxygen reduction electrode comprising
a mixed manganese oxide catalyst comprising an octahedral molecular
sieve structure and Mn.sub.2O.sub.3. Specifically, the active
catalyst material (of the oxygen reduction electrode) comprises a
mixture of manganese oxide having an octahedral molecular sieve
structure (such as, for example, cryptomelane) and a manganese
oxide having the structure of Bixbyite (i.e., Mn.sub.2O.sub.3) for
use as a cathode component of an electrochemical cell.
[0049] As further detailed elsewhere herein, the cryptomelane
material, or other octahedral molecular sieve structure, is
preferably heat-treated (for example, calcined) so that a portion
of the cryptomelane converts to a Bixbyite (Mn.sub.2O.sub.3) phase.
The inclusion of the Bixbyite phase with the octahedral molecular
sieve structure in the catalyst results in, for example, a higher
open circuit voltage, a higher closed circuit voltage, and/or a
higher stability of the cathode under storage.
[0050] Suitably, in a preferred embodiment, the octahedral
molecular sieve structure and the Mn.sub.2O.sub.3 are present in
the mixed catalyst in a ratio between about 1:9 and about 9:1, more
suitably between about 1:4 and about 4:1, more suitably between
about 1:3 and about 3:1, and even more suitably between about 1:2
and about 2:1. More suitably, the octahedral molecular sieve
structure and the Mn.sub.2O.sub.3 are present in the mixed catalyst
in a ratio between about 2:1 and 1:1, more suitably between about
1.5:1 and about 1:1 and even more suitably between about 1.27:1 and
about 1:1. Even more suitably, the octahedral molecular sieve
structure and the Mn.sub.2O.sub.3 are present in the mixed catalyst
in a ratio of about 1:1.
[0051] Another metal cation can be contained in the octahedral
molecular sieve structure. That is, a portion of the octahedra
which forms the framework of the octahedral molecular sieve may
comprise another metal cation in place of a fraction of the
manganese. The other metal cation(s), M.sup.n+, (where n is the
oxidation state of the cation) can be, for example, an alkaline
earth metal, a p-block metal, a lanthanide, or more preferably, a
transition metal. See U.S. Pat. No. 5,702,674 issued to O'Young et
al, which is hereby incorporated by reference as if set forth in
its entirety. Examples of useful metals for incorporating into the
octahedra include Mg, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Zr, Nb, Ga,
In, Ge, Sn, Pb, La, and combinations thereof. Preferred metals
include Ti, Zr, Sn, Co, Ni, Cu and Zn. To achieve incorporation of
metal cation(s), M.sup.n+, into the octahedra which form the
framework of the octahedral molecular sieve, the cation(s) can be
introduced into the reaction mixture during the preparation of the
octahedral molecular sieve structure, e.g., in the process of
Example 1 of U.S. Pat. No. 4,975,346, and/or in the preparation as
described hereinbelow. The metal cation to be contained in the
octahedral framework can be introduced in the reaction mixture in a
concentration effective to incorporate the metal(s) in a desired
proportion of the octahedra which forms the framework of the
molecular sieve structure during the synthesis. Any suitable ionic
compound (organic or inorganic) of the selected metal(s) can be
used which is sufficiently soluble provided that the anion does not
interfere with the other reactants or the course of reaction. For
example, metal ionic compounds, such as nitrates, sulfates,
perchlorates, alkoxides, acetates, oxides, and the like, can be
introduced into the reaction mixture, which generally results in
the incorporation of a metal cation into a portion of the octahedra
which forms the octahedral molecular sieve framework. Manganese
oxide contained in an octahedral molecular sieve structure which
has a metal other than Mn incorporated into its framework can be
referred to as "a manganese oxide octahedral molecular sieve
containing a secondary framework metal," e.g., "a manganese oxide
octahedral molecular sieve containing framework titanium;" or, more
specifically, where the octahedral molecular sieve is cryptomelane,
"cryptomelane containing framework titanium."
[0052] The air cathode additionally comprises a polymeric binder,
which can be a hydrophobic polymer such as polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVDF), fluorinated
ethylene-propylene polymer (FEP), perfluoroalkoxy resin (PFE, a
copolymer of tetrafluoroethylene and perfluorovinylethers),
ethylene-tetrafluoroethylene copolymer (ETFE),
polychlorotrifluoroethylene (PCTFE),
ethylene-chloro-trifluoroethylene copolymer (ECTFE), and polyvinyl
fluoride (PVF), styrene butadiene rubber (SBR), and combinations
thereof. The polymeric binder is included to bind the conductive
material and active catalyst together and impart
hydrophobicity.
[0053] The air cathode comprising the components listed above is
preferably conductive, porous, gas and liquid permeable, and has
both hydrophobic and hydrophilic character. An air cathode for use
in a metal-air cell requires both hydrophobic and hydrophilic
properties because the cathode must be sufficiently hydrophilic to
be wettable to allow hydroxyl ions to diffuse from the cathode
active material to the zinc anode surface where they may react with
the Zn, but it must also be sufficiently hydrophobic to avoid
flooding the cathode material with electrolytic solution, thus
causing a reduction in the rate of reaction by creating an
excessive barrier to the diffusion of molecular O.sub.2 through the
liquid phase to the cathode surface for reduction to hydroxyl ions
at the active catalyst layer. Without being bound to a particular
theory, it is thought that hydrophilic and hydrophobic surfaces and
passages are separately distributed throughout the bulk of the
cathode material. Advantageously, the cathode may be formed in a
relatively thin layer within the cathode can, e.g., in the range
between about 5 mils (about 120 .mu.m) and about 20 mils (about 500
.mu.m).
[0054] In constructing a cell in which the air cathode comprises
the active catalyst material of the present invention, the air
cathode is electrically insulated from the anode material by a
separator. The separator is a porous, permeable material containing
electrolytic solution which allows hydroxyl ions formed by the
reduction of oxygen to diffuse to the anode material, but prevents
electrical contact between the air cathode and the anode. The
separator may be constructed of polypropylene or polyethylene
treated to make it wettable with concentrated KOH, PVA film, or ion
exchange resin film. Alternatively, the separator may be a fibrous
paper having voids into which electrolytic solution may penetrate.
The fibers may also absorb the electrolytic solution. In another
alternative, the separator may be a fibrous paper in combination
with a polymer film. In yet another alternative, the separator may
comprise an anion exchange membrane composed of quaternary amine
cationic sites. Preferably, the separator is thin, having a
thickness less than about 3.0 mils (about 75 .mu.m) so that the
cell may comprise predominantly electrode materials, but the
separator is suitably at least about 0.5 mils (about 12 .mu.m) so
as to prevent electrical contact between the cell electrodes.
[0055] The anode may comprise a porous zinc mass, comprising
loosely or tightly compacted zinc powder, or an open cell porous
zinc monolith, with an ionically conductive electrolytic solution
in the pores. Where the anode comprises a zinc powder, individual
zinc particles may or may not be porous, but the anode mass has
porosity from the interstitial volume defined by a matrix
comprising a continuous network of substantially contiguous zinc
particles. A complementary matrix comprising a continuous
interpenetrating network of preferably gelled electrolytic solution
fills the interstitial volume defined by the zinc powder mass. Zinc
is a preferred anode material because it is stable in an alkaline
electrolyte and achieves a good rate of discharge. Preferably, the
total mass of zinc in the zinc-air cell is related to the capacity
output of the cell. The mass is limited by the size of the cell,
with cells having standard sizes PR44, PR48, PR41, and PR70 being
most currently preferred. Preferably, the zinc is a powder, having
an average particle size distribution between about 125 .mu.m and
about 500 .mu.m. The zinc powder typically has a bulk density
between about 2.5 g/cm.sup.3 and about 3.5 g/cm.sup.3.
[0056] The ionically conductive electrolytic solution typically
comprises potassium hydroxide, sodium hydroxide, or lithium
hydroxide in an aqueous solvent, with potassium hydroxide currently
preferred because of its high conductivity and favorable
equilibrium vapor pressure. Suitably, the concentration of the
electrolyte is between about 20% by weight and about 40% by weight
of the electrolytic solution, more suitably from about 30% by
weight and about 35% by weight. Preferably, the ionically
conductive electrolytic solution is gelled using suspending agents
such as, for example, carboxymethylcellulose (CMC), polyacrylic
acid, and sodium polyacrylate. The suspending agent is typically
present in the electrolytic solution at a concentration between
about 0.05% by weight and about 1% by weight, suitably about 0.1%
by weight in the electrolytic solution. The ionically conductive
electrolyte preferably comprises a surfactant, for example, an
oxazoline surfactant. See U.S. Pat. No. 3,389,1454, U.S. Pat. No.
3,336,145, in U.S. Pat. No. 4,536,300, in U.S. Pat. No. 5,758,374,
in U.S. Pat. No. 5,407,500, and in U.S. Pat. No. 6,927,000, all of
which are hereby incorporated by reference as if set forth in their
entirety. Suitably, the surfactant is present at a concentration
between about 0.1% by weight and about 1% by weight, more suitably
about 0.2% by weight in the electrolytic solution.
[0057] At the anode, hydroxyl ions, react with and oxidize the zinc
anode material according to the following half-cell reaction:
2Zn+4OH.sup.-=>2ZnO+2H.sub.2O+4e.sup.- E.sup.o=1.25V v. S.H.E.
(2)
The zinc oxide further reacts with the hydroxyl ions present in the
electrolyte to form hydrated zincate ions.
[0058] Combining the half cell reactions which occur at the anode
and at the cathode yields the overall reaction and the theoretical
cell voltage of a zinc-air cell:
O.sub.2+2Zn=>2ZnO E.sup.o=1.65V v. S.H.E. (3)
In conventional zinc-air cells, the factors affecting polarization
cited above, especially peroxide buildup, tend to lower the working
cell voltage below the theoretical value.
Catalytic Reduction of Oxygen
[0059] The novel active catalyst material and cathode of the
present invention are typically capable of operating at a higher
voltage for a given current density than a conventional manganese
oxide catalyzed cathode. The active catalyst material comprises
manganese oxide contained in an octahedral molecular sieve
structure. The manganese oxide contained in an octahedral molecular
sieve structure advantageously catalyzes reduction of peroxide
anions and decomposition of hydrogen peroxide. Peroxide anions are
intermediates in the reduction of oxygen to hydroxyl ions, which
may be shown according to the following hypothesized mechanism:
O.sub.2+e.sup.-=>O.sub.2..sup.- (1)
O.sub.2..sup.-+H.sub.2O+e.sup.-=>OOH.sup.-+OH.sup.- (2)
OOH.sup.-+H.sub.2O+2e.sup.-=>3OH.sup.- (3)
O.sub.2+2H.sub.2O+4e.sup.-=>4OH.sup.- (4)
Instead of reduction of the peroxide anion in step (3) to the
desired hydroxyl ion, the peroxide anion may be protonated to form
hydrogen peroxide which may decompose into water and adsorbed
oxygen according to the following reaction:
OOH.sup.-+H.sup.+=>HOOH (5)
HOOH=>H.sub.2O+1/2O.sub.2(ad) (6)
The adsorbed oxygen may then undergo additional reduction as show
in steps (1) and (2) above. Slow reaction kinetics at either step
(3) the reduction of peroxide anion or (6) the decomposition of
peroxide into water and oxygen may cause peroxides to buildup at
the electrode, thus limiting the achievable current density and
lowering the working voltage of the cell.
[0060] Advantageously, manganese oxide contained in an octahedral
molecular sieve structure is believed to exhibit higher catalytic
activity for steps (3) and (6) compared to conventional manganese
oxide compounds. Preferably, the atomic scale tunnels are formed by
a framework comprising a 2.times.2 arrangement of edge-shared
MnO.sub.6 octahedra and have approximate dimensions of 4.6
.ANG..times.4.6 .ANG.. Without being bound to a particular theory,
it is thought that the atomic scale tunnels have dimensions which
provide shape selectivity, i.e., geometric or energy state
preference for the adsorbed reactant or intermediate, in this
instance for peroxide anions, and advantageously catalyzes step (3)
the reduction of peroxide anions to hydroxyl ions. Metal exchange
of a portion of the potassium ion in the atomic scale tunnels of
cryptomelane, such as by Ni.sup.2+ has been reported to enhance
step (6) hydrogen peroxide decomposition. See Zhou et al., Journal
of Catalysis, 176, 321-328 (1998). Increased catalysis of both
steps (3) and (6) enhances the rate of formation of hydroxyl ions,
which oxidize zinc metal at the anode, thus allowing the full cell
to reflect the full voltage obtainable by the complete redox
reaction as shown below.
O.sub.2+2Zn=>2ZnO E.sup.o=1.65V v. S.H.E. (7)
[0061] The air cathode may additionally comprise other oxygen
reduction catalysts such as activated carbon and/or catalytically
active manganese oxide compounds. For example, the manganese oxide
compounds prepared in the method described in U.S. Pat. Nos.
5,308,711 and 5,378,562 to Passaniti et al. exhibit enhanced
catalytic activity for oxygen reduction.
Synthesis of Manganese Oxide Contained in an Octahedral Molecular
Sieve Structures
[0062] The synthesis of manganese oxide contained in an octahedral
molecular sieve structure has been reported. See DeGuzman, Roberto
N. et al., Chem. Mater. 1994, 6, 815-821 (the disclosure of which
is hereby incorporated in its entirety). See also Giovanili et al.,
Chimia, 35, (1981) 53, U.S. Pat. Nos. 5,597,944 and 5,702,674 to
O'Young et al., U.S. Pat. No. 6,486,357 to Suib et al., and U.S.
Pat. No. 4,975,346 to Lecerf et al. (the disclosures of which are
hereby incorporated in their entirety).
[0063] An exemplary preparation of cryptomelane starts with the
redox reaction of manganese sulfate (MnSO.sub.4) with potassium
permanganate (KMnO.sub.4) in aqueous acidic solution at a
temperature suitably greater than about 50.degree. C., more
suitably between about 60.degree. C. and about 70.degree. C. The
Mn.sup.2+ salt can be MnCl.sub.2, MnSO.sub.4, Mn(NO.sub.3).sub.2,
Mn(CH.sub.3COO).sub.2, as well as other known salts. The
permanganate can be provided as the sodium salt, potassium salt,
magnesium salt, calcium salt, barium salt, ammonium salt, as well
as other known salts. The solution can be acidified, preferably to
a pH less than about 4.5 using nitric acid, hydrochloric acid,
sulfuric acid, and strong organic acids. For the synthesis of
cryptomelane, the Mn sources are preferably manganese sulfate
(MnSO.sub.4) and potassium permanganate (KMnO.sub.4). The molar
concentration ratio of [MnO.sub.4.sup.-]/[Mn.sup.2+] can range from
between about 0.2 to about 3.0, suitably between about 0.2 and
about 1.4. For example, if the concentration of Mn.sup.2+ is about
1.0 M, then the concentration of the MnO.sub.4.sup.- is preferably
between about 0.2 M and about 1.4 M. The reaction typically
proceeds for about 1 hour to about 4 hours, suitably for about 2
hours with stirring. The MnSO.sub.4 solution is introduced into a
reaction vessel and maintained under agitation at a temperature
suitably in the range of about 60.degree. C. to about 70.degree. C.
The KMnO.sub.4 solution is added slowly to the reactor while the
temperature of the resulting mixture is preferably controlled in
the aforementioned range. Reaction of KMnO.sub.4 and MnSO.sub.4
yields cryptomelane, an octahedral molecular sieve in which
potassium ions reside in the atomic scale tunnels along with a
small amount of water.
[0064] To achieve an octahedral molecular sieve structure
comprising both manganese and another metal cation incorporated
into the octahedra which make up the framework of the octahedral
molecular sieve, another metal cation is introduced into the
reaction mixture during addition of the oxidizing agent, which in
the above preparation, is the permanganate. For example, a suitable
ionic compound of a metal such as Mg, Ti, V, Cr, Fe, Co, Ni, Cu,
Zn, Zr, Nb, Ga, In, Ge, Sn, Pb, La, and combinations thereof may be
added to the solution comprising potassium permanganate. Adding the
salt of the metal cation to the permanganate solution, which is
then added to the Mn.sup.2+ solution, is sufficient to incorporate
the metal cation into the octahedra which make up the framework of
the octahedral molecular sieve. In the preferred synthesis, the
added metal cation becomes preferentially incorporated into the
framework of the octahedral molecular sieve structure rather than
into the atomic scale tunnels. Without being bound to a particular
theory, it is thought that the counter cation of the preferred
alkali metal permanganate or other oxidizing agent, such as
potassium, preferentially resides in the atomic scale tunnels to
charge balance the manganese oxide contained in an octahedral
molecular sieve. Because the potassium or other counter cation
preferentially resides in the atomic scale tunnel, the added metal
cation such as one selected from the list above becomes
incorporated into the octahedral molecular sieve framework.
Suitably, manganese and another metal are charged to the reaction
solution in relative proportions approximating their proportions in
the octahedral molecular sieve structure. Suitably, the
concentration of the metal cation to be incorporated into the
octahedra is between about 5% and about 30% of the total
concentration of Mn, more suitably about 20% of the total
concentration of Mn. Suitably, the concentration of both the
Mn.sup.2+ and the Mn.sup.7+ sources are present at a ratio of
[MnO.sub.4.sup.-]/[Mn.sup.2+] in the reaction mixture for preparing
an octahedral molecular sieve with an octahedral metal cation can
be substantially the same as the ratio of
[MnO.sub.4.sup.-]/[Mn.sup.2+] in solution for preparing an
octahedral molecular sieve without another octahedral metal cation.
For example, if the ratio of [MnO.sub.4.sup.-]/[Mn.sup.2+] in the
preferred synthesis of an octahedral molecular sieve without
another octahedral metal cation is about 0.7, the ratio of
[MnO.sub.4.sup.-]/[Mn.sup.2+] is maintained at about 0.7 when
another metal cation is added to the reaction mixture to maintain.
In some syntheses, the ratio of [MnO.sub.4.sup.-]/[Mn.sup.2+] can
be altered if the metal cation added to the reaction mixture for
framework incorporation into the octahedral molecular sieve can
undergo reduction or oxidation during octahedral molecular sieve
synthesis. For example, added Fe.sup.2+ may be oxidized to
Fe.sup.3+ during the course of the octahedral molecular sieve
synthesis. Accordingly, the amount of MnO.sub.4.sup.- oxidizing
agent added can be increased relative to the amount of Mn.sup.2+,
which increases the ratio of [MnO.sub.4.sup.-]/[Mn.sup.2+].
Conversely, another metal cation added for framework incorporation
may undergo reduction and thus the amount of Mn.sup.2+ can be
increased relative to MnO.sub.4.sup.-, which decreases the ratio of
[MnO.sub.4.sup.-]/[Mn.sup.2+]. The concentration of the other metal
cation in the reaction mixture can be sufficient to synthesize an
octahedral molecular sieve structure in which between about 5% and
about 30% of the octahedra comprise another metal cation in place
of manganese, more suitably between about 5% and about 20% of the
octahedra comprise another metal cation in place of manganese, even
more suitably about 20% of the octahedral comprise another metal
cation in place of manganese.
[0065] Typically, while the reaction progresses, cryptomelane
precipitates out of the aqueous solution, and the precipitate is
characterized by a brownish-black color. Cryptomelane is sparingly
soluble. Upon completion of the reaction, the brownish-black
precipitate is filtered under vacuum and washed with distilled
water. The cryptomelane material typically has an average particle
size between about 0.01 .mu.m and about 10 .mu.m, a B.E.T. surface
area between about 50 m.sup.2/g and about 500 m.sup.2/g. The
material has a density between about 3 g/cm.sup.3 and about 4.5
g/cm.sup.3.
[0066] Alternatively, the manganese oxide contained in an
octahedral molecular sieve structure may be synthesized without
metal ions residing in the tunnels. See Example 1 of Lecerf et al.
in which the starting material is MnSO.sub.4 oxidized with ammonium
persulfate.
[0067] The cryptomelane material or other octahedral molecular
sieve structure is preferably heat-treated, for example, calcined,
to evaporate surface and lattice water and to attain a reduced
manganese oxidation state. Calcination and/or drying may occur in
an oven, for example, at a temperature suitably between about
95.degree. C. and about 650.degree. C., more suitably between about
100.degree. C. and about 500.degree. C., even more suitably between
about 150.degree. C. and about 400.degree. C. In a preferred
embodiment of the present disclosure, however, calcination may
occur at a temperature suitably between about 450.degree. C. and
about 650.degree. C., more suitably between about 500.degree. C.
and about 600.degree. C., more suitably between about 525.degree.
C. and about 575.degree. C., and even more suitably at about
550.degree. C. The material may be calcined and/or dried typically
between about 1 and about 5 hours, more suitably between about 2
hours and about 4 hours, even more suitably between about 3 hours
and about 4 hours. Calcination may occur in air, in an oxygen
atmosphere, or in a nitrogen atmosphere, but in the interest of
costs, air is preferred. Calcination may occur under a vacuum, but
in the interest of costs, ambient pressure is preferred.
[0068] Typically, the average oxidation state of the manganese in
heat-treated cryptomelane is no greater than about 4.2, suitably no
greater than about 3.85, more suitably between about 3.6 and about
3.85, and still more suitably between about 3.7 and about 3.85. In
a preferred embodiment of the present disclosure, however, the
average oxidation state of the manganese in the heat-treated mixed
catalyst is less than about 3.7, less than about 3.6, less than
about 3.5 or even less than about 3.4, suitably between about 3.1
and 3.7, more suitably between about 3.2 and 3.6, more suitably
between about 3.25 and 3.5, and even more suitably about 3.3.
[0069] The average oxidation state represents a mixed valence of Mn
ions between Mn(IV), Mn(III), and Mn (II) with higher valences
included in small amounts. The calcined cryptomelane typically has
a total water content less than about 10 wt. %, suitably less than
about 4 wt. %, more suitably less than about 2 wt. %.
[0070] The Mn oxidation state can be controlled at least in part by
the ratio of Mn(VII) and Mn(II), e.g., KMnO.sub.4 and MnSO.sub.4
used in the redox reaction by which the manganese oxide contained
in an octahedral molecular sieve structure is formed. The oxidation
state of Mn as contained in the precipitate can be reduced by high
temperature calcination. Lower oxidation states are preferred
because the higher Mn.sup.3+ content not only creates a mixed
valence oxide with higher electronic conductivity, but also
increases the efficacy of the oxide for catalysis of oxygen
reduction. Advantageously, high temperature calcination also lowers
the material's water content, improves its crystallinity, generally
increases particle size, decreases surface area, and removes other
impurities such as mixed sulfur containing oxides.
[0071] After synthesis of the manganese oxide contained in an
octahedral molecular sieve structure, the cations in the atomic
scale tunnels can be exchanged for another metal cation. If the
synthesis placed no cations in the tunnels, metal cations can be
placed in the atomic scale tunnels. After the preferred synthesis,
the octahedral molecular sieve atomic scale tunnels contain
potassium ions. The potassium ions can be replaced by exposing the
octahedral molecular sieve to a solution comprising a metal cation,
preferably as its nitrate salt. Accordingly, the preparation of a
cryptomelane catalyst of the present invention can additionally
comprise the step of exchanging a portion of the potassium ions
with other metal ions. Nickel-exchanged cryptomelane has high
catalytic activity for peroxide decomposition. Preferably, the
exchange can be carried out by percolating over the dried
cryptomelane an aqueous solution comprising nickel ions in a
concentration between about 10% and about 50% with a volume of
solution such that the ratio of nickel cation in aqueous solution
to potassium cation in the cryptomelane is between about 2:1 and
about 10:1. The extent of exchange may be controlled by adjusting
the concentration of the nickel ion solution, by the exposure time,
and the temperature. The exchange can be carried out from room
temperature, i.e., about 25.degree. C., up to about 100.degree. C.,
preferably at room temperature. By controlling the concentration
and exposure time, the extent of exchange may be limited to
replacing about 10% of the potassium ions in the atomic scale
tunnels. Alternatively, conditions can be varied to exchange
between about 40% and about 50% of the potassium ions with nickel
cations.
[0072] The additional treatments, such as calcinations, nickel or
other metal exchange, and metal-framework incorporation, may
advantageously improve the voltage of the cathode using the
manganese oxide contained in an octahedral molecular sieve
structure of the present invention.
Preparation of Air Cathode Comprising Manganese Oxide Contained in
an Octahedral Molecular Sieve
[0073] Manganese oxide contained in an octahedral molecular sieve,
such as, for example, heat-treated (optionally nickel-exchanged
and/or optionally containing framework titanium) cryptomelane,
whose preparation is described generally above and more
specifically in the examples below, can be used as the active
catalyst material of an air cathode for use in a zinc-air cell.
Referring to FIG. 3, a flow chart is presented showing, generally,
the steps in fabricating an air cathode according to the present
invention. Starting from the top of the flow chart, Mn.sup.2+ and
Mn.sup.7+ salts are reacted in the presence of acid, such as nitric
acid, to yield cryptomelane 40. Potassium ions resident in the
atomic scale tunnels of the cryptomelane active catalyst material
are optionally exchanged 42 for another metal cation, preferably
nickel. Thereafter, the active catalyst material is calcined in the
manner described above. Optionally, an additional metal cation can
be added to the reaction mixture. The addition of the metal cation
can result in the incorporation of the metal cation into a portion
of the octahedra which forms the framework of the octahedral
molecular sieve structure. Preferably, the metal cation is
Ti.sup.4+.
[0074] Heat-treated (optionally nickel-exchanged and/or optionally
containing framework titanium) cryptomelane, carbon black, and
hydrophobic polymeric binder (PTFE suspension) are then combined 44
into a catalyst slurry in preferably aqueous medium. The added
carbon and hydrophobic polymer serve to wet proof 46 the mix to
prevent flooding in the finished cathode. The carbon black
additionally imparts electrical conductivity to the finished
cathode. In preparing the catalyst slurry, heat-treated (optionally
nickel-exchanged and/or optionally containing framework titanium)
cryptomelane can be mixed with carbon black in water for between
about 15 min and about 120 min, with agitation, at room
temperature. Next, dry PTFE powder or a suspension of PTFE or other
binder may be added to the combined cryptomelane/carbon black
slurry to yield a mixture typically comprising between about 60%
and about 90% by weight cryptomelane, between about 1% and about
10% by weight carbon, and between about 1% and about 40% by weight
binder, the balance water. As alternatives to
polytetrafluoroethylene (PTFE), the binder may be selected from
among other polymeric materials such as polyvinylidene fluoride
(PVDF), fluorinated ethylene polymer (FEP), perfluoroalkoxy resin
(PFE, a copolymer of tetrafluoroethylene and perfluorovinylethers),
ethylene-tetrafluoroethylene copolymer (ETFE),
polychlorotrifluoroethylene (PCTFE),
ethylene-chloro-trifluoroethylene copolymer (ECTFE), polyvinyl
fluoride (PVF) styrene butadiene rubber (SBR), and combinations
thereof. The catalyst slurry is agitated for about 30 min to about
120 min.
[0075] A second mix, the carbon support mix, is prepared by
combining 48 activated carbon, graphite, and polymeric binder (PTFE
suspension) into an aqueous slurry. The activated carbon is an
oxygen reduction catalyst, and the graphite is added to impart
electrical conductivity to the finished cathode. The carbon support
slurry typically comprises between about 50% and about 90% by
weight carbon and between about 5% and about 50% by weight binder,
in aqueous medium. This slurry may be blended about 30 min,
filtered under vacuum, washed with distilled water, and dried at a
temperature between about 90.degree. C. and about 120.degree. C.
for about 2 hours to about 12 hours in an oven under an atmosphere
of air.
[0076] Alternatively, the carbon support mix can be prepared to
comprise another manganese oxide compound. Accordingly, the final
active cathode material can comprise manganese oxide contained in
an octahedral molecular sieve structure and another manganese oxide
compound. Optionally, the manganese oxide compound can be prepared
according to the methods disclosed by Passaniti et al. (U.S. Pat.
Nos. 5,308,711 and 5,378,562.) Briefly, those patents disclose
preparing a manganese oxide compound by adding activated carbon to
a solution comprising alkali metal permanganate to produce an
aqueous permanganate solution having activated carbon slurried
therein. The activated carbon reduces the permanganate to a
manganese oxide compound having catalytic activity for the
reduction of oxygen. The preparation of the catalytically active
manganese oxide compound can occur by contacting alkali metal
permanganate with activated carbon in a weight ratio of alkali
metal permanganate to carbon between about 0.01:1 and about 0.2:1,
more suitably between about 0.02:1 and about 0.1:1, and even more
suitably between about 0.04:1 and about 0.08:1 to prepare an
aqueous solution having a volume of about 5 mL of water per 1 g of
total solids. The reaction occurs over about 10 minutes with
mixing. The other ingredients of the carbon support mix may be
added directly into the aqueous solution comprising the manganese
oxide compound upon completion of the redox reaction between the
potassium permanganate and activated carbon. In a finished cathode
comprising a manganese oxide contained in an octahedral molecular
sieve structure and another manganese oxide compound, such as, for
example, the manganese oxide compound of Passaniti et al., between
about 5% and about 40% of the manganese oxide can be contained in
the octahedral molecular sieve structure, more preferably between
about 15% and about 35%, and the remainder as another manganese
oxide compound. The manganese in the manganese oxide compound other
than the octahedral molecular sieve can have an oxidation state
ranging anywhere from about 2 to about 4. Accordingly, the total
manganese, i.e., the manganese both contained in the octahedral
molecular sieve and the manganese in the other manganese oxide
compound, in the cathode suitably has an average oxidation state
between about 2.1 and about 4.0.
[0077] In yet another alternative, both the catalyst mix and the
carbon support mix can be prepared by a dry mixing process. The
catalyst mix can be prepared by mixing cryptomelane, carbon black,
and hydrophobic polymeric binder (PTFE) with a mechanical mixer,
such as a food processor. The carbon support mix can be separately
prepared by combining activated carbon, graphite, and polymeric
binder (PTFE) with a mechanical mixer.
[0078] In the preferred wet process, the catalyst slurry and carbon
support mix are combined 50 to make a cathode mix, which is mixed
and then preferably filtered, washed 52 with distilled water, and
dried 54 at a temperature suitably between about 75.degree. C. and
about 120.degree. C. between about 2 hours and about 12 hours in an
oven under an atmosphere of air. The dried carbon support slurry
has an approximate bulk density between about 0.20 g/cm.sup.3 and
about 0.50 g/cm.sup.3. In the alternative where both catalyst mix
and carbon support mix are prepared by dry mixing, combining the
two mixes yields a dry cathode mix which does not require further
drying.
[0079] Preferably, the cathode mix is a combination of two
mixtures, prepared separately, as described above. In an
alternative method of preparation, all ingredients of the cathode
mix may be combined directly into a single mixture, which can be a
slurry or dry mix. The two mixture preparation method
advantageously improves the balance of hydrophobicity and
hydrophilicity in the air cathode. Further, the two-mixture
preparation maintains a separation of the manganese oxides having
octahedral molecular sieve structure separate and the activated
carbon, thus inhibiting a redox reaction between the activated
carbon and the manganese oxides having octahedral molecular sieve
structures from occurring before the catalyst slurry and carbon
support mixture are combined.
[0080] The dry cathode mix is preferably further mixed and kneaded
to ensure intimate mixing of the constituents to attain consistency
and uniformity of hydrophobic and hydrophilic regions within the
mix. The dried cathode mix is then coarsely milled to break up
large clumps. The isolated particles are of sufficient size to
press together into a sheet, which is difficult to achieve for
large clumps and dust particles. The sieved particles are fed to
rollers and rolled 56 into a cathode sheet, which is referred to as
an active catalyst layer 58. The rolled material typically has a
density between about 0.8 g/cm.sup.3 and about 1.4 g/cm.sup.3.
Suitably, the cathode sheets are very thin, e.g., between about 2
mils (about 50 .mu.m) and about 20 mils (about 500 .mu.m) in
thickness, to allow the cell to contain as much Zn anode material
as possible, given the dimensional constraints of the particular
cell.
[0081] These cathode sheets are pressed 60 into a current collector
substrate, which is a cross bonded screen having Ni strands woven
therein, the Ni screen conveniently having a 40 mesh size. The
current collector substrate transmits current to the cathode can
and minimizes the voltage drop between the cathode/electrolytic
solution interface and the positive terminal of the cell. The
cathode sheet rolling pressure and pressure to press the cathode
sheet into the current collector substrate are relatively low such
that the pressed cathode material maintains high porosity.
[0082] The resulting cathode sheet is laminated 62 with an air
diffusion layer. An air diffusion layer of hydrophobic material
such as, e.g., a pure PTFE membrane, is laminated on the side of
the cathode sheet opposite to the side of the cathode sheet which
was pressed into the current collector.
[0083] Finally, a thin polypropylene or other dielectric separator
sheet is glued 64 onto the side of the cathode sheet which was
pressed into the conductive current collector. This cathode sheet
is now ready to be punched to yield individual cathodes for use in
air-metal cells. The cathode sheet is simultaneously punched with
an additional PTFE membrane sheet adjacent to the air diffusion
layer along with a non-woven paper layer adjacent to the dielectric
separator sheet. The sheet may be punched 66 to yield disc-shaped
carbon-based air cathodes loaded with heat-treated (and optionally
nickel-exchanged) cryptomelane. Depending upon the size of the
cell, the cathode sheet may typically be punched to yield air
cathodes having a diameter between about 5 mm and about 13 mm.
EXAMPLES
[0084] The following examples further illustrate the invention.
Example 1
Synthesis of Cryptomelane
[0085] To prepare cryptomelane, manganese sulfate monohydrate
(MnSO.sub.4.H.sub.2O; 88 g; available from Alfa/Aesar, Ward Hill,
Mass.) was stirred into a nitric acid solution that had been
prepared by mixing concentrated nitric acid (30 mL) with deionized
water (300 mL). The resulting acidic Mn salt solution was
constantly stirred with a magnetic stir bar.
[0086] The temperature of the solution was raised to 60 C, and an
aqueous solution containing 5.89% by weight potassium permanganate
(1000 g) was slowly added with constant stirring (KMnO.sub.4 from
Alfa/Aesar, Ward Hill, Mass.). The redox reaction between Mn.sup.2+
from MnSO.sub.4 and Mn.sup.7+ from KMnO.sub.4 resulted in the
formation of a brownish-black precipitate. The slurry was stirred
at 60.degree. C. for four hours. Finally, the cryptomelane product
was isolated by filtration under vacuum and washing with distilled
water (500 mL).
Example 2
X-Ray Diffraction Characterization of Cryptomelane
[0087] To determine whether the synthesis successfully yielded
cryptomelane, samples of the material prepared according to the
method of Example 1 were heat treated and prepared for X-ray
diffraction analysis. A first sample was heat-treated at
300.degree. C. and loaded onto a powder sample holder for an X-ray
diffractometer and subjected to X-ray diffraction analysis. A
second sample was heat-treated at 95.degree. C. and loaded onto a
powder sample holder for an X-ray diffractometer and subjected to
X-ray diffraction analysis. X-ray diffraction confirmed that the
resultant material had cryptomelane structure. See FIG. 4 for X-ray
diffraction patterns of cryptomelanes heat treated at 300.degree.
C. 68 and heat treated at 95.degree. C. 70 compared against
standard cryptomelane-M-K.sub.2,Mn.sub.8O.sub.16 (44-1386).
Approximate values for the major peak positions for the sample heat
treated at 300.degree. C. are presented in Table 1 below. The exact
numerical values obtained by X-ray diffraction analysis of a
crystal structure can vary depending upon the X-ray diffractometer
and analytical conditions. Therefore, values may vary somewhat from
the values presented in Table 1.
TABLE-US-00001 TABLE 1 Major Peak Positions from the X-Ray
Diffraction of Cryptomelane Heat Treated at 300.degree. C. Angle, d
value, Intensity, Intensity, 2-Theta .degree. Angstrom count % 12.7
7.0 41.9 43 18.1 4.9 39.1 40 25.6 3.5 8.2 8 28.7 3.1 59.3 61 37.8
2.4 96.8 100 39.1 2.3 8.13 8 42.0 2.2 36 37 49.9 1.8 38.1 39 56.1
1.6 16.8 17 56.4 1.6 21.7 22 56.7 1.6 16.4 17 60.2 1.5 39.4 41 65.4
1.4 36.6 38 69.5 1.4 16.9 17
[0088] From these XRD data, it was determined that cryptomelane has
unit cell parameters: a=9.848 .ANG..+-.0.02 .ANG., b=2.862
.ANG..+-.0.02 .ANG., c=9.843 .ANG..+-.0.06 .ANG., and
p=90.08.degree..+-.0.4.degree., when calculated as a monoclinic
unit cell.
Example 3
Nickel-Exchanged Cryptomelane
[0089] Exchange of potassium ions residing within the cryptomelane
tunnels with nickel ions was performed by suspending a sample of
the solid material prepared according to the method of Example 1 in
0.1M NiNO.sub.3 solution (1 Liter) available from Alfa/Aesar, Ward
Hill, Mass. The resultant nickel-exchanged cryptomelane was
filtered under vacuum and washed with distilled water (1000
mL).
[0090] Alternatively, the exchange of potassium ions with nickel
ions was carried out by passing 0.1M NiNO.sub.3 solution through a
fixed bed comprising particulate cryptomelane prepared according to
the method of Example 1. For example, a sample of the precipitate
prepared according to the method of Example 1 was recovered by
filtration and the NiNO.sub.3 solution passed through the solid
filter cake under vacuum filtration. The sample was heat treated at
300.degree. C. and prepared for X-ray diffraction analysis.
Referring now to FIG. 5, an X-ray diffraction pattern of
nickel-exchanged cryptomelane heat treated at 300.degree. C. 72 may
be compared against the PDF of
cryptomelane-M-K.sub.2-xMn.sub.8O.sub.16 (44-1386) and against an
empirically determined X-ray diffraction pattern of unexchanged
cryptomelane heat treated at 300.degree. C. 77.
Example 4
Cryptomelane Calcination
[0091] Samples (50 grams each) were taken from the solid filtered
cake of nickel-exchanged cryptomelane prepared according to the
method of Example 3 and subjected to heat treatment at temperatures
of 95.degree. C., 120.degree. C., and 400.degree. C. for four
hours. Additionally, samples of unexchanged cryptomelanes prepared
according to the method of Example 1 were subjected to high
temperature drying and/or calcination at temperatures of about
120.degree. C., 300.degree. C., 500.degree. C., 600.degree. C., and
700.degree. C.
[0092] Referring to FIG. 6, the calcined nickel-exchanged
cryptomelane samples were prepared for X-ray diffraction, and X-ray
diffraction patterns of each sample dried at 95.degree. C. (80 in
FIG. 6), 120.degree. C. (82 in FIG. 6), and calcined at 400.degree.
C. (84 in FIG. 6) were obtained. As can be seen from the XRD
patterns, the structure remains intact during the calcination
process, even at calcination carried out at temperatures as high as
400.degree. C. In fact, the crystallinity of the structure improves
with increasing calcination temperature, as can be seen by the
narrower peaks in the X-ray diffraction patterns.
[0093] The change in chemical and physical properties with
calcination temperature for unexchanged cryptomelane is shown in
Table 2. The change in chemical and physical properties with
calcination temperature for non-exchanged cryptomelane is shown in
Table 3. The data indicate a trend toward lower manganese oxidation
state with increasing calcination temperature. Without being bound
by a particular theory, it is thought that high temperature
calcination causes the thermal decomposition of the cryptomelane
and release of oxygen. It is apparent that the surface area of the
samples tends to decrease and the particle size of the samples
tends to increase with increasing calcination temperature. However,
at very high temperature calcination, such as above about
600.degree. C., the oxidation state decreases even further and
there is a reversal in the trend toward smaller surface area and
larger particle size. Without being bound by a particular theory,
it is thought that high temperature calcination results in a phase
transition from cryptomelane to Mn.sub.2O.sub.3.
TABLE-US-00002 TABLE 2 Chemical and Physical Properties of
Unexchanged Cryptomelane Catalyst at Different Calcination
Temperatures Calcination and/or Drying Manganese Surface Mean
Temperature Oxidation Area Particle (.degree. C.) State
(m.sup.2/mg) size (.mu.m) 95 3.95 89.22 1.941 300 3.82 63.86 2.077
500 3.59 17.08 2.513 600 3.19 42.28 1.963 700 3.14 8.49 1.376
TABLE-US-00003 TABLE 3 Chemical and Physical Properties of
Ni-exchanged Cryptomelane Catalyst at Different Calcination
Temperatures Calcination and/or Drying Manganese Surface Mean
Temperature Oxidation Area Particle (.degree. C.) State
(m.sup.2/mg) size (.mu.m) 95 4.07 113.63 3.096 120 3.93 114.42
2.467 400 3.82 89.06 3.534
Example 5
Stability Testing of Ni-Exchanged Cryptomelane
[0094] The stability of Ni-exchanged cryptomelane in basic solution
was tested. A sample of Ni-exchanged cryptomelane prepared
according to the method of Example 4 but heat treated 300.degree.
C. (1 g) was added to KOH electrolytic solution (100 mL). The
solution was stirred for 16 hours. After stirring, the solution was
filtered under vacuum, washed with deionized water (100 mL), and
dried at a temperature between about 65.degree. C. and about
100.degree. C. for about 60 min in an oven under an atmosphere of
air. The sample of Ni-exchanged cryptomelane heat treated at
300.degree. C. and further subjected to the treatment described
above was prepared for X-ray diffraction. Another sample of
Ni-exchanged cryptomelane heat treated at 300.degree. C. but not
exposed to the KOH electrolytic solution was also prepared for
X-ray diffraction. The X-ray diffraction patterns of both
KOH-exposed 90 and unexposed 92 Ni-exchanged cryptomelanes heat
treated at 300.degree. C. are shown in FIG. 7. As can be seen from
the patterns, Ni-exchanged cryptomelane is stable to this treatment
against visible decomposition, crystal structure loss, and
performance.
Example 6
Preparation of Cryptomelane Containing Framework Titanium
[0095] Manganese sulfate monohydrate (MnSO.sub.4.H.sub.2O; 77 g;
available from Alfa/Aesar, Ward Hill, Mass.) was stirred into a
nitric acid solution that had been prepared by mixing concentrated
nitric acid (30 mL) with deionized water (300 mL). The resulting
acidic Mn salt solution was constantly stirred with a magnetic stir
bar.
[0096] The temperature of the solution was raised to 60.degree. C.
An aqueous solution containing potassium permanganate and titanium
oxide sulfate (prepared by dissolving 51.5 g KMnO.sub.4 and 26.95 g
TiOSO.sub.4.H.sub.2O in concentrated H.sub.2SO.sub.4 and diluting
to 1000 g with water, both reactants from Alfa/Aesar, Ward Hill,
Mass.) was slowly added with constant stirring. The redox reaction
between Mn.sup.2+ from MnSO.sub.4 and Mn.sup.7+ from KMnO.sub.4
resulted in the formation of a brownish-black precipitate with
Ti.sup.4+ ions in the framework. The slurry was stirred at
60.degree. C. for two hours. Finally, the cryptomelane containing
framework titanium was isolated by filtration under vacuum and
washing with distilled water (500 mL). X-ray diffraction and
chemical analysis were used to identify the structure and chemical
formula of the structure.
[0097] Alternatively, the cryptomelane containing framework
titanium is also prepared by adding predetermined quantity of
soluble titanium oxide sulfate in the manganese sulfate solution to
which potassium permanganate solution is added.
Example 7
Preparation of Dried Cathode Mix with Nickel-Exchanged
Cryptomelane
[0098] A catalyst slurry was prepared by adding nickel-exchanged
cryptomelane (30 g, calcined at 400.degree. C.) prepared according
to the method of Example 4 and carbon black (7 g; available from
Cabot Corp., Billerica, Mass.) to DI water (100 mL). The slurry was
mixed with a magnetic stir bar for 10 minutes. T-30 Teflon.RTM.
suspension (3.3 g; from DuPont) was added to the slurry. The
resulting catalyst slurry was mixed for an additional 30
minutes.
[0099] A carbon support mix was prepared by mixing PW activated
carbon (45 g; from Calgon, Corp., Pittsburgh, Pa.), SF6 graphite (3
g; from Timcal, Westlake, Ohio), and T-30 Teflon.RTM. suspension
(21.4 g) in DI water (200 mL). This carbon support slurry was
thoroughly blended, filtered, washed, and dried in an oven at
95.degree. C. The resulting cake was blended in a dry mixture,
sieved, and stored.
[0100] To prepare the cathode mix, the dried and granulated carbon
support mix was added to the catalyst slurry and mixed together for
10 minutes. The combined mixture was filtered, washed, and dried in
an oven at 95.degree. C. overnight to yield the cathode mix.
Example 8
Preparation of Dried Cathode Mix with Heat-Treated Cryptomelane
[0101] Alternatively, the dried cathode mix was prepared using
heat-treated cryptomelane prepared according to the method of
Example 4. The preparation of the dried cathode mix is
substantially the same as shown in Example 7, provided that
unexchanged, heat-treated cryptomelane is added to make the
catalyst slurry instead of nickel-exchanged cryptomelane. In this
example, the cathode was also prepared with a catalyzed carbon
support comprising catalytically active manganese oxide compounds
prepared according to the method described by Passaniti et al.
(U.S. Pat. Nos. 5,308,711 and 5,378,562.)
Example 9
Cathode Fabrication
[0102] Two dry cathode mixes prepared according to the methods
described in Examples 7 and 8 were separately dry mixed and kneaded
in a blender to ensure intimate mixing of the constituents. The
cathode mixes were then milled and fed to rollers to roll into two
cathode sheets having a density between about 0.8 g/cm.sup.3 and
about 1.4 g/cm.sup.3.
[0103] These cathode sheets were pressed into a current collector
substrate, which was a cross bonded screen having Ni strands woven
therein (from GDC, Hanover, Pa.) having a 40 mesh size and 5 mil
(about 120 .mu.m) wires. A strip of each of these cathode sheets
were used for half-cell testing as described in Example 10.
[0104] An air diffusion layer of pure PTFE membrane was laminated
on the side of the cathode sheet opposite to the side of the
cathode sheet which was pressed into the current collector.
Finally, a thin polypropylene separator sheet (from Hoechst
Celanese, Charlotte, N.C.) was glued onto the current collector.
According to this method, two cathode sheets were prepared: (a) a
first cathode sheet comprising heat-treated cryptomelane in
combination with catalytically active manganese oxide compounds,
and (b) a second cathode sheet comprising nickel-exchanged
cryptomelane catalyst. The cathode sheet is simultaneously punched
with an additional PTFE membrane sheet adjacent to the air
diffusion layer along with a non-woven paper (from Mitsubishi
Corp., New York, N.Y.) adjacent to the dielectric separator sheet.
These punched sheets are then used in the construction of the full
cell.
Example 10
Half Cell Testing of Heat-Treated and Nickel-Exchanged Cryptomelane
Air Cathodes
[0105] The fabricated cathode strips of Example 9 were tested in
half cells to obtain information about the cathode without
interference from the anode reaction or the air diffusion layer. A
first cathode strip contained heat-treated cryptomelane in
combination with catalytically active manganese oxide compounds. A
second cathode strip contained nickel-exchanged cryptomelane
catalyst. The cathode strips served as the working electrodes. A
piece of zinc wire and nickel mesh served as the reference and
counter electrodes, respectively. A solution of potassium hydroxide
and zinc oxide (31% KOH and 2% ZnO) in DI water was used as the
electrolytic solution. The current collector side of the cathode
strips was exposed to the electrolytic solution, while the catalyst
mix layer was exposed to the air.
[0106] The electrochemical measurements were performed using a
Solartron (SI 1287) Instrument. Current v. Voltage (polarization)
behavior of the heat-treated cryptomelane cathode and the
nickel-exchanged cryptomelane cathode was obtained by scanning
through the voltage from the open circuit voltage (OCV) to 0.5
volts against the reference electrode at 1 mV per second. The
voltage of the cathode was measured with reference to the zinc
electrode at various current densities with intermittent rest
period. All the results are compared to a conventional cathode.
FIG. 8 shows polarization curves for cathodes loaded with both
heat-treated cryptomelane and nickel-exchanged cryptomelane in
comparison to a conventional cathode. The half cell voltage is
consistently higher for both cathodes loaded with cryptomelanes at
voltages greater than 1.1 volts.
[0107] Referring to FIG. 9, it can be seen that the discharge
voltages of the cathodes loaded with heat-treated cryptomelane and
nickel-exchanged cryptomelane achieve consistently higher discharge
voltages in comparison to the conventional cathode at current
densities of 0.1 mA/cm.sup.3 (100 in FIG. 9), 5 mA/cm.sup.3 (102 in
FIG. 9), 20 mA/cm.sup.3 (104 in FIG. 9), and 50 mA/cm.sup.3 (106 in
FIG. 9). These data are summarized in TABLE 4 and show that the
first cathode strip comprising heat-treated cryptomelane in
combination with catalytically active manganese oxide compounds and
the second cathode strip comprising nickel-exchanged cryptomelane
catalyst exhibited discharge voltages increased by about 25 mV or
more at various current densities.
TABLE-US-00004 TABLE 4 Half Cell Voltages at Different Current
Densities Current Density (mA/cm.sup.2) OCV 0.1 5 20 50 Present
Cathode 1.394 V 1.392 V 1.344 V 1.300 V 1.239 V Cathode loaded
1.412 V 1.411 V 1.371 V 1.326 V 1.252 V with Ni-exchanged
Cryptomelane Cathode loaded 1.423 V 1.421 V 1.382 V 1.334 V 1.255 V
with Heat-Treated Cryptomelane and Catalytically Active Manganese
Oxide Compounds
Example 11
Mixture of Manganese Oxides Having an Octahedral Molecular Sieve
Structure and Bixbyite
[0108] As noted above, the cryptomelane can be prepared by
filtering, washing and drying solids obtained from the reaction of
a KMnO.sub.4 and MnSO.sub.4.H.sub.2O solution at 70.degree. C. FIG.
16 depicts thermogravimetric analysis (TGA)curves for cryptomelane
in air as well as in argon. The cryptomelane samples were heated at
5.degree. C./min. For the TGA curve in air, after the initial
weight loss (that can be ascribed to loss of outbound water), the
slope of the weight loss was almost constant from about 160.degree.
C. to about 520.degree. C. The slope increased at the point where
the weight loss became higher with the rise in temperature. The
flattening of the curve then occurred at about 620.degree. C.,
signaling a completion of the reaction. As shown in FIG. 16, the
inflection points occurred at lower temperatures for the
cryptomelane sample heated under argon.
[0109] The results of the samples of cryptomelane that were heated
at temperatures between about 450.degree. C. and about 600.degree.
C. are shown below. Specifically, the effects of temperature of
calcination on the manganese oxidation state, the calculated
chemical formula, the surface area and the particle size are
represented in Table 5.
TABLE-US-00005 TABLE 5 Calcination and/or Drying BET Mean
Temperature, Oxidation Chemical Surface Particle .degree. C. State
Formula Area, m.sup.2/g Size, .mu.m 120 3.90 K.sub.0.11MnO.sub.2.00
114.6 1.731 350 3.76 K.sub.0.11MnO.sub.1.93 85.4 1.655 400 3.72
K.sub.0.10MnO.sub.1.92 76.41 1.739 450 3.66 K.sub.0.11MnO.sub.1.89
66.3 1.730 500 3.47 K.sub.0.11MnO.sub.1.79 45.07 1.801 550 3.33
K.sub.0.11MnO.sub.1.72 28.06 1.839 600 3.18 K.sub.0.11MnO.sub.1.64
20.07 1.901 650 3.17 K.sub.0.11MnO.sub.1.64 16.68 1.995
[0110] As shown in Table 5, as the calcination temperature
increases from about 450.degree. C. to about 650.degree. C., the
manganese oxidation state and the B.E.T. surface area decrease,
while the mean particle size increases.
[0111] Further, as shown in FIG. 10, the emergence of peaks for the
Bixbyite (Mn.sub.2O.sub.3) phase occurs at about 500.degree. C. and
corresponds to the increase in slope of the manganese oxidation
state versus the temperature plot. The growth of the peaks of the
Bixbyite phase is observed for samples calcined at 550.degree. C.
and even more for the samples calcined at 600.degree. C. This
observation is consistent with the change in the slope of the
thermal gravimetric analysis curve represented in FIG. 16 and the
change in the oxidation state of manganese in the samples of FIG.
10.
[0112] From these results, it can be observed that the weight loss
associated with the samples heated between 120.degree. C. and
450.degree. C. does not cause a change in the structure of the
cryptomelane. Meanwhile, the decrease in the manganese oxidation
state is evidence of the loss of oxygen from the structure of
cryptomelane and the creation of oxygen vacancies. The Bixbyite
phase is formed at about 500.degree. C. and at temperatures above
that. The partial phase transition of the cryptomelane with
manganese ions having an oxidation state of 4 (i.e., Mn.sup.+4
ions) to Mn.sub.2O.sub.3 having Mn.sup.+3 ions is observed in the
abrupt change in the manganese oxidation state shown in FIG. 10 at
500.degree. C.
[0113] FIG. 18 depicts the powder X-ray diffraction patterns for
the heat-treated samples. Again, there is little change in patterns
for the samples treated at temperatures between about 120.degree.
C. and about 450.degree. C. The emergence of peaks occurs for the
Bixbyite (Mn.sub.2O.sub.3) phase at about 500.degree. C. and grows
at about 550.degree. C. This observation is consistent with the
change in slope of the TGA curves in FIG. 16 and the change in the
manganese oxidation state in the samples.
[0114] The amount of Mn.sub.2O.sub.3 phase present in the sample
can be calculated both from chemical formula and by Rietveld
analysis of the X-ray diffraction data. For example, FIG. 11A
depicts the Rietveld analysis for the X-ray diffraction data for a
sample calcined at 500.degree. C. From this analysis, one having
ordinary skill in the art can determine that the Bixbyite phase
constitutes about 31% of the mixed catalyst, while the cryptomelane
constitutes about 69% of the mixed catalyst. FIG. 11B shows that at
550.degree. C., the Bixbyite phase constitutes about 56% of the
mixed catalyst, while the cryptomelane constitutes about 44% of the
mixed catalyst. FIG. 11C shows that at 600.degree. C., the Bixbyite
phase constitutes about 75% of the mixed catalyst, while the
cryptomelane constitutes about 25% of the mixed catalyst.
[0115] The amount of Bixbyite (Mn.sub.2O.sub.3) phase in the mixed
catalyst can also be calculated from the chemical formula
calculated from the chemical analysis data, as indicated in Table
6.
TABLE-US-00006 TABLE 6 Composition by Drying Reitveld Composition
from Temperature, Chemical Analysis Chemical Formula .degree. C.
Formula KMn8O16 Mn2O3 KMn8O16 Mn2O3 105 K.sub.0.11MnO.sub.2.00
100.0 0.0 100 0 350 K.sub.0.11MnO.sub.1.93 100.0 0.0 100 0 400
K.sub.0.10MnO.sub.1.92 100.0 0.0 100 0 450 K.sub.0.11MnO.sub.1.89
100.0 0.0 100 0 500 K.sub.0.11MnO.sub.1.79 68.6 31.5 70.1 29.9 550
K.sub.0.11MnO.sub.1.72 56.0 44.0 49.3 50.7 Composition by
Composition from Reitveld Analysis Chemical Formula 600
K.sub.0.11MnO.sub.1.64 25.2 74.8 26.6 73.4 650
K.sub.0.11MnO.sub.1.64 23.5 76.5 25.4 74.6
Example 12
Electrochemical Performance of Mixed Catalysts
[0116] The mixed catalyst samples of Example 11 were mixed with
activated carbon, conductive carbon and PTFE and rolled into
gas-diffusion electrodes. The electrodes were then tested in a
half-cell with zinc wire as the reference in 33% KOH solution
containing 2% zinc oxide. The electrodes were discharged at 10
mA/cm.sup.2 and the recovery of voltage was monitored under an open
circuit. The discharge and recovery curves are depicted in FIG.
12.
[0117] As is shown in FIG. 12, the samples calcined at 500.degree.
C., 550.degree. C. and 600.degree. C., respectively, performed the
best under load. The samples made at 450.degree. C. and 650.degree.
C. have lower voltage under load. Additionally, the recovery
voltage of the 450.degree. C. sample was lower than the other
samples and the recovery voltage of the 650.degree. C. sample was
lower than the recovery voltage of the 500.degree. C., 550.degree.
C. and 600.degree. C. samples.
[0118] FIG. 13 depicts the running voltage at three minutes into
discharge and FIG. 14 depicts the recovery voltage at five minutes
off load. As shown in FIGS. 13 and 14, respectively, the highest
values for cell voltages were obtained for the 500.degree. C.,
550.degree. C. and 600.degree. C. samples.
[0119] FIG. 15 depicts the polarization curves of the samples. The
voltage was scanned at 1 mV/sec and the current was then measured.
Similar to the discharge and the recovery samples, the best
performance was obtained with samples calcined at 500.degree. C.,
550.degree. C. and 600.degree. C.
[0120] FIGS. 19-22 depict the electrochemical performance of the
samples tested in a full cell. FIG. 19 depicts the increase in open
circuit voltage of the mixed catalyst calcined at 500.degree. C.
Specifically, FIG. 19 shows that the open circuit voltage of the
mixed catalyst calcined at 500.degree. C. was 28 mV higher than
that of the pure cryptomelane catalyst made at 120.degree. C. and
300.degree. C., respectively. Furthermore, the mixed catalyst
calcined at 500.degree. C. had an open circuit voltage of about 57
mV more than the state of the art control cathode.
[0121] FIGS. 20 and 21 depict the higher discharge voltage for the
cells comprising the mixed catalyst calcined at 500.degree. C. than
the pure cryptomelane catalysts made at 120.degree. C. or
300.degree. C. and a state of the art control cathode. The
discharge curve in FIG. 20 is 620 ohm constant load and the
discharge curve in FIG. 21 is 374 ohm constant load, with the cells
being discharged at 50% relative humidity.
[0122] FIG. 22 depicts the discharge curves for the sample cells
after they have been stored at 60.degree. C. for twenty days. The
storage time of twenty days at 60.degree. C. represents
approximately twelve months of storage time at room temperature on
a shelf. FIG. 22 shows that the sample cells with the mixed
catalyst made at 500.degree. C. produced higher voltage for the
full range of discharge than the cells with cathodes containing
pure cryptomelane made at 120.degree. C. or 300.degree. C. It is
also believed that the higher stability of the cathode or catalyst
can be traced back to the surface area of the catalysts. For
example, the mixed catalyst material made at 500.degree. C. has a
surface area of 41.5 m.sup.2/g while the pure cryptomelane catalyst
made at 120.degree. C. or 300.degree. C. has surface areas of 114.6
m.sup.2/g and 85.4 m.sup.2/g, respectively.
[0123] In view of the above, it will be seen that the several
objects of the invention are achieved and other advantageous
results attained.
[0124] When introducing elements of the present invention or the
preferred embodiment(s) thereof, the articles "a", "an", "the", and
"said" are intended to mean that there are one or more of the
elements. The terms "comprising", "including", "containing", and
"having" are intended to be inclusive and mean that there may be
additional elements other than the listed elements.
[0125] As various changes could be made in the above without
departing from the scope of the invention, it is intended that all
matter contained in the above description and shown in the
accompanying drawings shall be interpreted as illustrative and not
in a limiting sense.
* * * * *